An internal gear and an external gear having a number of teeth differing from a number of teeth of the internal gear are provided, and the external gear is disposed in a position in which the external gear is surrounded by the internal gear while meshing with the internal gear. The external gear rotates with respect to the internal gear when the external gear is caused to revolve orbitally around an axis line of the internal gear while the external gear and the internal gear are maintaining the meshing state, i.e. when the external gear is caused to rotate eccentrically around the axis line of the internal gear. An inner meshing planetary gear type gear transmission that utilizes this phenomenon has been developed, and is disclosed for example in Japanese Patent Application Publication No. 2001-187945. The conventional gear transmission comprises an internal gear, an external gear, a carrier, a crank member, and bearings.
The external gear has a number of teeth differing from a number of teeth of the internal gear. The external gear is capable of rotating while revolving orbitally around an axis line of the internal gear while in a meshing state with the internal gear. The external gear is disposed in a position surrounded by the internal gear. The external gear has a center through hole at its center, and a plurality of offset through holes is formed at positions offset from its center.
The carrier comprises column-shaped members and a pair of supporting members. Each of the column-shaped members passes through the respective offset through hole of the external gear. The pair of supporting members respectively supports each ends of the column-shaped members. Further, the pair of supporting members is supported by the internal gear in a state allowing the pair of supporting members to rotate around the axis line of the internal gear. A space is maintained between each of the column-shaped members and the respective offset through hole that corresponds thereto. That is, the carrier allows the external gear to revolve orbitally around the axis line of the internal gear, and the carrier rotates around the axis line of the internal gear together with the rotation of the external gear.
The crank member comprises a shaft, an eccentric rotation member, and an input gear. The shaft passes through the center through hole of the external gear. The eccentric rotation member has an axis line of rotation at a position offset from an axis line of the shaft and is fitted in the center through hole of the external gear. The shaft is capable of rotating with respect to the carrier. The input gear is fixed to the shaft, and causes the shaft to rotate.
The bearings are each disposed between the respective supporting member of the carrier and the shaft of the crank member. Further, the bearings are disposed as a pair at positions between which the eccentric rotation member of the crank member is interleaved. The shaft of the crank member is supported by the pair of bearings such that the shaft is capable of rotating with respect to the carrier and is not capable of moving in an axial direction of the carrier.
A pair of additional bearings is further disposed between the internal gear and the supporting members of the carrier. The carrier is supported by the pair of additional bearings such that the carrier can rotate with respect to the internal gear.
Since the above configuration is provided, when the input gear of the crank member rotates, the shaft of the crank member rotates, and the eccentric rotation member of the crank member rotates eccentrically. When the eccentric rotation member of the crank member rotates eccentrically, the external gear rotates with respect to the internal gear while revolving orbitally around the axis line of the internal gear. When the external gear rotates, the carrier rotates with respect to the internal gear.
In the inner meshing planetary gear type gear transmission, the crank member must be supported so as to be capable of rotating with respect to the pair of supporting members of the carrier and so as to be incapable of moving in the axial direction of the carrier. In the conventional gear transmission, the crank member is supported so as to be capable of rotating with respect to the carrier and so as to be incapable of moving in the axial direction of the carrier by disposing a pair of tapered roller bearings between the pair of supporting members and the shaft of the crank member. Further, the carrier must be supported so as to be capable of rotating around the axis line of the internal gear. In the conventional gear transmission, the carrier is supported so as to be capable of rotating around the axis line of the internal gear by disposing a pair of tapered roller bearings between the internal gear and the pair of supporting members of the carrier. In the conventional gear transmission, the number of parts constituting the gear transmission is large, and the cost of the gear transmission therefore is high.